1. a) Field of the Invention
The present invention relates to an imaging optical system which is to be used in combination with a solid stage image pickup device such as a CCD or a CMOS. The present invention relates to an imaging optical system which is to be utilized in a compact camera, a monitor camera or the like to be incorporated, for example, with a digital still camera, a digital video camera, a portable telephone or a personal computer.
2. b) Description of the Prior Art
There have recently been prevailing electronic cameras which photograph objects using solid-state image pickup devices such as a CCD and a CMOS in place of silver salt films. Out of such electronic cameras, image pickup unit which are to be incorporated with portable computers, portable telephones and the like in particular require compactness and light weight.
As an image pickup optical system which is to be used in such an image pickup unit, there is conventionally known an optical system consisting of a lens or two lenses. As apparent from an aberration theory, however, such an optical system cannot correct curvature of field, thereby making it hopeless to have high optical performance.
On the other hand, a CCD hinders a micro lens from exhibiting a sufficient light condensing capability when an offaxial light bundle emerging from an imaging optical system is incident at too large an angle relative to an image surface. The CCD therefore produces a phenomenon that brightness is extremely changed from a central portion of an image to a marginal portion of the image. Since this phenomenon is associated with a location of an exit pupil of the optical system, the location of the exit pupil is important for design of the optical system. Furthermore, the exit pupil is in a conjugate relation to an aperture stop. A location of the aperture stop is therefore important for an optical system which consists of a small number of lenses.
As optical system which is configured taking this point into consideration, there is a type of optical system in which a stop is disposed before a first lens (lens on a most object side) or between the first lens and a second lens. Known as conventional examples of this type optical system are those which are disclosed by patent literatures mentioned below:                [Patent literature No. 1] Japanese Patent Kokai Publication No. Hei 5-188284        [Patent literature No. 2] Japanese Patent Kokai Publication No. Hei 9-288235        [Patent literature No. 3] Japanese Patent Kokai Publication No. 2001-83403        [Patent literature No. 4] Japanese Patent Kokai Publication No. Hei 11-5227        
Out of these optical systems, each of the optical system disclosed by literatures No. 1 and No. 2 uses a first positive lens which is a biconvex lens and a second negative lens which is a biconcave lens. This optical system therefore allows optical performance to be remarkably degraded when the lenses are eccentric from each other. In other words, the optical system requires a greatly high assembling precision for exhibiting high performance.
Furthermore, the optical system disclosed by the literature No. 3 uses a first positive lens which has a concave surface on the object side and a circumferential portion of the lens hinders the optical system from having a short total length.
Furthermore, the optical system disclosed by the literature No. 4 uses a first positive lens which has a meniscus shape convex on the object side and a second lens which has a meniscus shape convex on the image side. Accordingly, this optical system overcomes a difficulty of enlargement in a field angle.